1. Field of the Invention
The present invention relates to a metal oxide semiconductor (MOS) device and a method for fabricating the same.
2. Description of Related Art
Metal oxide semiconductor (MOS) transistor is one of the most common elements used in many different semiconductor devices, such as memories, image sensors or displays. A typical MOS includes a silicon oxide dielectric layer, a gate conductive layer, and heavily doped source/drain contact regions. Along with decrease in linewidth of semiconductor devices, dimensions thereof are reduced. Due to the reduction in the gate width of the typical MOS, the channel length of the MOS is consequentially reduced.
As the channel length reduces, applying mechanical stress to the channel is proposed to effectively change mobility of electrons and holes in the channel, thereby increasing operating speed of the transistor. A conventional technique has been provided with using silicon germanium (SiGe) epitaxy material as a major component of the source/drain contact regions in the transistor. As compared with characteristics of silicon material, germanium has larger atomic volume and can apply a compressive stress toward the channel. Thus, the mobility of the holes can be enhanced in the source/drain contact regions majorly made of silicon germanium (SiGe), and thereby device performance can be improved.
In the fabrication process of the device, an epitaxial growth rate of silicon germanium (SiGe), however, varies in accordance with different density of surrounding patterns of the device to be formed. Accordingly, the thickness of the resultant silicon germanium (SiGe) is non-uniform (i.e. so-called loading effects), and issues regarding mismatches of the device performance arise therefrom.